Tetra-Lateral Position Sensing Detector

ABSTRACT

The present invention is directed to a position sensing detector made of a photodiode having a semi insulating substrate layer; a buffered layer that is formed directly atop the semi-insulating substrate layer, an absorption layer that is formed directly atop the buffered layer substrate layer, a cap layer that is formed directly atop the absorption layer, a plurality of cathode electrodes electrically coupled to the buffered layer or directly to the cap layer, and at least one anode electrode electrically coupled to a p-type region in the cap layer. The position sensing detector has a photo-response non-uniformity of less than 2% and a position detection error of less than 10 μm across the active area.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/774,958, filed on May 6, 2010, which relies on U.S.Provisional Application No. 61/177,329, filed on May 12, 2009. Inaddition, the present application is related to U.S. Pat. No. 6,815,790,which is herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to the field of position sensingdetectors, and more specifically, to a tetra-lateral semiconductor basedposition-sensing photodiode, having improved photo responsenon-uniformity.

BACKGROUND OF THE INVENTION

The demand for precise detection of the position of incident light iscrucial for a variety of applications, such as automatic focusing,position-sensing, movement monitoring, mirror alignment, distortion andvibration measurements, and for use within photocopy machines, facsimilemachines, automatic lighting systems, articulated robotic beam deliverysystems, optical switches and remote controls, optical range finders,laser displacement sensors, computer tomography and cameras. Eachapplication requires an efficient and effective optoelectronic device toascertain the correct coordinates.

Conventionally, various instruments, such as small discrete detectorarrays or multi-element sensors, are used to detect the position ofincident light. However, photodiode-based position-sensing detectors(PSDs) offer higher position resolution, higher speed response andgreater reliability than other solutions. Photodiode-based PSDs convertan incident light spot into continuous position data and aremanufactured from semiconductors such as silicon materials. Siliconphotodiodes, essentially active solid-state semiconductor devices, areamongst the most popular photo detectors. In addition, siliconphotodiodes detect the presence or absence of minute light intensitiesthereby facilitating correct measurement on appropriate calibration.

The abovementioned silicon photodiode have substantial disadvantages,however. Due to short cut-off wavelength of silicon materials,photodiodes manufactured from silicon are not suitable for applicationsthat involve longer wavelengths such as in communication systems or eyesafe detection applications. Therefore, PSDs employing materials otherthan silicon, such as indium-gallium-arsenide (InGaAs)/indium-phosphide(InP) which have a cut-off wavelength suitable for photo-detection, havebeen developed.

Conventionally, PSDs employing InGaAs/InP are classified as eitherone-dimensional or two-dimensional. Two-dimensional PSDs are more usefulin ascertaining position than one-dimensional PSDs because they candetect movement in two dimensions and provide adequate details aboutcoordinates. Two-dimensional PSDs are further divided into duo-lateraland tetra-lateral position sensing detectors.

Duo-lateral PSDs typically have two anode electrodes on the front sideand two cathode electrodes on the backside. While duo-lateral PSDs haveexcellent position linearity, they tend to have relatively high darkcurrent, low speed response and complicated application of reverse bias.Duo-lateral PSDs can be disadvantageous in that they are expensive tomanufacture as they requires front-to-back mask alignment capability.

Accordingly, there is a need for a tetra-lateral PSD that can bemanufactured at a lower cost than a duo-lateral PSD. In addition, thereis a need for a tetra-lateral PSD InGaAs/InP photodiode that can be usedin a longer wavelength region.

SUMMARY OF THE INVENTION

The present invention is directed toward a position sensing detectorcomprising a photodiode having an active area, said photodiodecomprising a semi insulating substrate layer; a buffered layer, whereinsaid buffered layer is formed directly atop the semi-insulatingsubstrate layer; an absorption layer, wherein said absorption layer isformed directly atop the buffered layer substrate layer; a cap layer,wherein said cap layer is formed directly atop the absorption layer; aplurality of cathode electrodes electrically coupled to said bufferedlayer; and at least one anode electrode electrically coupled to a p-typeregion in said cap layer, wherein said detector is capable of detectingeye-safe wavelengths. Eye-safe wavelengths are at least from 1.3-1.55μm. The buffered layer comprises InP. The absorption layer comprisesInGaAs. The cap layer comprises InP. The photo response non-uniformityis less than 1 micron across said active area. The position detectionerror, in both the X and Y direction is on the order of 100 μm acrossthe active area. The photodiode comprises four cathode electrodes. Eachindividual cathode is positioned parallel to the other cathodes and inopposing corners of the photodiode. The photodiode further comprises ananti-reflective layer positioned atop the cap layer. The p-type regionin said cap layer is formed by diffusing a region of said cap layer witha suitable dopant to create said p-type region. The dopant is zinc.

In another embodiment, the present invention is directed toward aposition sensing detector comprising a photodiode having an active area,said photodiode comprising a semi insulating substrate layer; a bufferedlayer, wherein said buffered layer is formed directly atop thesemi-insulating substrate layer; an absorption layer, wherein saidabsorption layer is formed directly atop the buffered layer substratelayer; a cap layer, wherein said cap layer is formed directly atop theabsorption layer, wherein a p-n junction is formed between said caplayer and said absorption layer; a plurality of cathode electrodeselectrically coupled to said buffered layer; and at least one anodeelectrode electrically coupled to said cap layer. The buffered layercomprises at least one of InGaAs or InP. The absorption layer comprisesat least one of InGaAs or InP. The cap layer comprises at least one ofInGaAs or InP. The position detection error, in both the X and Ydirection is on the order of 100 μm across the active area. Thephotodiode comprises four cathode electrodes. The p-n junction is formedby diffusing a region of said cap layer with a suitable dopant. The p-njunction is formed by diffusing a region of said absorption layer with asuitable dopant.

In another embodiment, the present invention is directed toward aposition sensing detector comprising a photodiode having an active area,said photodiode comprising a semi insulating substrate layer; a bufferedlayer, wherein said buffered layer is formed directly atop thesemi-insulating substrate layer; an absorption layer, wherein saidabsorption layer is formed directly atop the buffered layer substratelayer; a cap layer, wherein said cap layer is formed directly atop theabsorption layer, wherein a p-n junction is formed between said caplayer and said absorption layer; a plurality of cathode electrodeselectrically coupled to said cap layer; and at least one anode electrodeelectrically coupled to said cap layer.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will beappreciated, as they become better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings:

FIG. 1 is a perspective view of a conventional one-dimensional positionsensing detector;

FIG. 2 shows a top, cross-sectional view of a two-dimensionaltetra-lateral position sensitive detector;

FIG. 3 shows a perspective side view of one embodiment of thetetra-lateral position sensitive detector of the present invention;

FIG. 4 shows a top, perspective view of the tetra lateral positionsensitive detector of the present invention;

FIG. 5 shows an equivalent circuit of the tetra-lateral position sensingdetector of the present invention;

FIG. 6 is a cross-sectional view of another embodiment of thetwo-dimensional position sensing detector of the present invention;

FIG. 7 a is a front, cross-sectional view showing the starting materialfor fabricating one embodiment of the position sensing detector (PSD) ofthe present invention;

FIG. 7 b is a front, cross-sectional view of the PSD, depicting thefabrication step of deposition of PECVD silicon nitride on a front side;

FIG. 7 c is a front, cross-sectional view of the PSD, depicting thefabrication step of coating the front side with a photoresist layer;

FIG. 7 d is a front, cross-sectional view of the PSD, depicting thefabrication step of p+ mask lithography on the front side;

FIG. 7 e is a front, cross-sectional view of the PSD, depicting thefabrication step of p+ Zn diffusion on the front side;

FIG. 7 f is a front, cross-sectional view of the PSD, depicting thefabrication step of depositing PECVD silicon nitride, anti-reflectivelayer and photoresist;

FIG. 7 g is a front, cross-sectional view of the PSD, depicting thefabrication step of mask lithography for opening an anode window;

FIG. 7 h is a front, cross-sectional view of the PSD, depicting thefabrication step of coating the front side with a photoresist layer;

FIG. 7 i is a front, cross-sectional view of the PSD, depicting thefabrication step of anode metal lift-off mask lithography on the frontside;

FIG. 7 j is a front, cross-sectional view of the PSD, depicting thefabrication step of evaporating metal on the front side;

FIG. 7 k is a front, cross-sectional view of the PSD, depicting thefabrication step of photoresist lift-off on the front side;

FIG. 7 l is a front, cross-sectional view of the PSD, depicting thefabrication step of coating the front side with a photoresist layer;

FIG. 7 m is a front, cross-sectional view of the PSD, depicting thefabrication step of mask lithography for opening a cathode window on thefront side;

FIG. 7 n is a front, cross-sectional view of the PSD, depicting thefabrication step of coating the front side with a photoresist layer;

FIG. 7 o is a front, cross-sectional view of the PSD, depicting thefabrication step of cathode metal lift-off mask lithography on the frontside;

FIG. 7 p is a front, cross-sectional view of the PSD, depicting thefabrication step of evaporating metal on the front side;

FIG. 7 q is a front, cross-sectional view of the PSD, depicting thefabrication step of photoresist lift-off on the front side; and

FIG. 8 illustrates a plot of a plurality of test measurements ofposition sensing error obtained by using the PSD of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed towards a tetra-lateralposition-sensing detector (hereinafter, “PSD”) comprising an InGaAs/InPphotodiode and/or photodiode array that can be used in a longerwavelength region. Specifically, the present invention is directedtowards a PSD for detecting light, and more specifically to atetra-lateral PSD comprised of a photodiode array fabricated from aInP/InGaAs/InP substrate, which can be used in the 800 nm to 3000 nmwavelength region.

In addition, the present invention is directed towards a photodiodearray, wherein each photodiode has a total of one anode electrode andfour cathode electrodes, all on the front side, which can bemanufactured at a lower cost than a duo-lateral PSD. More specifically,each photodiode comprises four cathode electrodes, whereby eachindividual cathode is positioned parallel to the other cathodes and inopposing corners of the photodiode.

The present invention is directed towards multiple embodiments. Thefollowing disclosure is provided in order to enable a person havingordinary skill in the art to practice the invention. Language used inthis specification should not be interpreted as a general disavowal ofany one specific embodiment or used to limit the claims beyond themeaning of the terms used therein. The general principles defined hereinmay be applied to other embodiments and applications without departingfrom the spirit and scope of the invention. Also, the terminology andphraseology used is for the purpose of describing exemplary embodimentsand should not be considered limiting. Thus, the present invention is tobe accorded the widest scope encompassing numerous alternatives,modifications and equivalents consistent with the principles andfeatures disclosed. For purpose of clarity, details relating totechnical material that is known in the technical fields related to theinvention have not been described in detail so as not to unnecessarilyobscure the present invention.

The present invention comprises a novel PSD chip structure that may beapplied to a plurality of fields of use and may be implemented usingvarious materials. In one embodiment, the PSD chip structure of thepresent invention comprises an anode metal in electrical communicationor contact with a P+ diffused cap layer, and a set of four cathodesparallel to the four edges of the chip and in electrical contact orcommunication with the undiffused N-type InP cap layer. The photodiodechip further comprises a first set of electrodes electrically coupled tothe buffered layer, and a second electrode placed parallel to the firstelectrode and electrically coupled to the cap layer. The PSD chipstructure is implemented using, individually or in combination, silicon,gallium-arsenide, indium-gallium-arsenide, indium-phosphide, germanium,mercury-cadmium-telluride layers, indium-arsenide-phosphide (InAsP) orother suitable semiconductor materials known to persons of ordinaryskill in the art. Further, the PSD chip structure is implemented suchthat the semiconductor layers are p-type doped or n-type doped, anddiffused as appropriate.

In one embodiment, the tetra lateral photodiode of the present inventionis fabricated on a InP/InGaAs/InP as a semiconductor starting material.The use of InGaAs/InP enables a photo-detection error of about 10 μm orless across the active area.

FIG. 1 is a perspective view of a conventional one-dimensional positionsensing detector. The position sensing detector 100 consists of auniform resistive layer formed on one or both surfaces of ahigh-resistivity semiconductor substrate. The active area of theposition sensitive detector 100 has a P-N junction that generatescurrent, by means of photovoltaic effect, which acts as a signal. Undera photovoltaic effect, the photodiodes are either under zero bias orreverse bias and the light falling on the diode causes a voltage todevelop across the device, leading to current in the forward direction.Generally, the forward and breakdown voltages at a current of 1 mAmpsand 1 μAmps range from 0.8V to 1.1V, and 10V to 26V, respectively. Apair of electrodes 105 a, 105 b is formed on both ends of top resistivelayer, in conjunction with another single electrode 110 at the bottomlayer, for extracting position signals.

FIG. 2 shows a top, cross-sectional view of a two-dimensionaltetra-lateral position sensitive detector in one embodiment of thepresent invention. The tetra-lateral, position-sensing detector 200comprises a first side 205 having a wire bonding pad for anode 230. Inaddition, the tetra-lateral PSD 200 further comprises second side 210,third side 215, and fourth side 220. The first side 205, second side210, third side 215, and fourth side 220 further comprise wire bondingpads for cathodes 235 a, 235 b, 235 c, and 235 d respectively. The anodepad 230 of the position sensitive detector 200 is in electrical contactwith a doped P-type InP cap layer (not shown) and cathode pads 235 a,235 b, 235 c, and 235 d are in electrical contact with an N-type InP caplayer (not shown).

In one embodiment of the present invention the first side 205, secondside 210, third side 215, and fourth side 220 of position sensitivedetector are on the order of 2.070×2.070 mm. In an embodiment of thepresent invention, the wire bonding pads for cathode, 235 a through 235d, are on the order of 0.15 mm×0.15 and the wire bonding pad for anode230 is on the order of 0.125 mm×0.2 mm. It should be noted herein thatwhile exemplary dimensions are listed for the wire bonding pads, thedesign can be modified in alternate embodiments, with the requirementthat the wire bonding pads should be large enough for convenient andeffective wire bondages.

In addition, tetra-lateral PSD 200 also comprises active area 225. Theactive area 225 receives light, converts it into photocurrents andtransfers it to a plurality of electrodes for ascertaining positioncoordinates of the incident light. In one embodiment of the presentinvention, the active area 225 of the position sensitive detector 200 ison the order of 2 mm×2 mm. It should be noted herein that the activearea 225 can be of any dimension, including, but not limited to 2 mm×3mm, 3 mm×3 mm, and 3 mm×6 mm, among other dimensions.

FIG. 3 shows a perspective side view of one embodiment of thetetra-lateral position sensitive detector of the present invention. Inone embodiment, the tetra lateral position sensitive detector 300 iscuboidal comprising a plurality of sides, also referred to as faces orfacets. The cuboid-shaped position sensitive detector 300 has sixdistinct facets: proximate 305, distant 310, top 315, bottom 320, left325, and right 330. The position sensitive detector 300 further includestwo sets of electrodes on the top facet 315. A first electrode 331functions as anode and a second set of electrodes 335 a, 335 b, 335 cand 335 d function as cathodes. In one embodiment the chip thickness ofthe position sensitive detector 300 is 0.200 (+/−) 0.015 mm. Persons ofordinary skill in the art would appreciate that the presentdesignation/dimension of position sensitive detector is not limited tothe description provided herein and can be adjusted to suit otherdesign, fabrication, and functional specifications. The chip can be ofany thickness in alternate embodiments.

FIG. 4 shows a top, perspective view of the tetra lateral positionsensitive detector 400 of the present invention. The four electrodes 435a, 435 b, 435 c and 435 d function as a cathode and the fifth electrode430 functions as an anode. In one embodiment, inter-electrode resistanceis measured at the operational voltage. In one embodiment of the presentinvention, the inter-electrode, and more specifically, inter-cathoderesistance along the horizontal axis and the vertical axis, at a reversebias of −5V, ranges from 2 to 2.5 KOhm. In one embodiment of the presentinvention, the inter-cathode resistance along the horizontal axis andthe vertical axis is 2.5 KOhm.

During operation, when light falls on active area 405 of the positionsensitive detector 400, photocurrent is generated which flows from thepoint of light incidence through the resistive layer to the electrodes430 and 435 a, 435 b, 435 c, and 435 d. The photocurrent generated isinversely proportional to the resistance between the incident light spotand the electrodes. When the input light spot is exactly at the devicecenter, current signals having equal strength are generated. When thelight spot is moved over the active area 405, the amount of currentgenerated at the electrodes determines the exact light spot position ateach instant of time since the electrical signals are proportionatelyrelated to the position of the light spot from the center. In oneembodiment of the present invention the tetra-lateral PSD comprises asingle resistive layer, where the photocurrent is divided into two partsfor one-dimensional sensing or, in the alternative, four parts fortwo-dimensional sensing.

FIG. 5 shows an equivalent circuit 500 of the tetra-lateral positionsensing detector of the present invention, shown as 400 in FIG. 4. Thevalues of position resistance Rxx and position resistance Ryy are set bythe thickness and doping concentration of the buffered layer.Photocurrent is divided into 4 parts through the same resistive layer(buffered layer) and collected as position signals from the fourelectrodes 535 a, 535 b, 535 c, and 535 d. Also, as shown in FIG. 5, Pis the current generator, D is ideal diode, Cj is junction capacitance,and R_(sh) is shunt resistance. In one embodiment of the presentinvention, the shunt resistance at −10 mV ranges from 1 to 8 MOhms andthe capacitance at 0V and at −5V ranges from 322 to 900 pico farads (pF)and 176 to 375 pF, respectively.

The electrodes 535 a, 535 b, 535 c, 535 d extract a first currentcomponent and the electrode 530 extracts a second current component.Both such current component values are then used to determine thecoordinates of the light spot based on appropriate equations as evidentto those of ordinary skill in the art.

Thus, the position of centroid of the incident light spot is indicatedalong with generating electrical output signals proportional to thedisplacement from the center. The input light beam may be of any sizeand shape.

In various embodiments, the responsivity of the PSD at a wavelength of1300 nm ranges from 0.85 to 0.959 A/W and at a wavelength of 1550 nmranges from 0.95 to 1.15 A/W. In one embodiment of the presentinvention, the minimum responsivity of the PSD at a wavelength of 1300nm is 0.90 A/W and at a wavelength of 1550 nm is 0.95 A/W. Theresponsivity of a position sensitive detector is a measure of thesensitivity to light, and is the ratio of the photocurrent to theincident light power at a given wavelength. According to an aspect ofthe present invention, the position detection error, in both the X and Ydirection is on the order of 100 μm across the active area.

FIG. 6 is a cross-sectional view of another embodiment of thetwo-dimensional position sensing detector 600 of the present invention.In one embodiment, an InP buffered layer 620 is formed over asemi-insulating substrate layer 610. An InGaAs absorption layer 630 isfurther formed over the buffered layer 620 and an InP cap layer 640 isthen formed over the absorption layer 630. The InP layer 640 is used tokeep surface dark current low, since InP is a wide bandgapsemiconductor.

In one embodiment, two sets of output electrodes (four cathodes and oneanode) that correspond to two dimensions are used. A first set ofelectrodes 660 a and 660 b (shown), 660 c and 660 d (not shown) areeither electrically coupled to the InP buffered layer 620 or directly tothe surface of the cap layer 640, and function as cathodes. It should benoted herein that the cathodes 660 a, 660 b, 660 c, and 660 d are formedfrom a deep well etch process to achieve metal contact with the bufferedlayer. It is preferred, however, in the present invention, as describedwith respect to FIG. 7 q that the cathode metal contacts the surface ofthe cap layer 640 only. Electrodes 660 a and 660 b are substantiallyparallel to each other and are each disposed near opposite ends of thePSD 600. Similarly, electrodes 660 c and 660 d are parallel to eachother and are disposed on the other end of the PSD 600. An electrode 650is electrically coupled to the p-type region 680 of the InP layer 640and functions as anode.

In one embodiment of the present invention, InGaAs layer 630 functionsas an i-layer. An anti-reflective layer 670, such as for example,silicon nitride, is preferably positioned over the InP layer 640 as ameans to control reflection and to passivate the surface of thejunction. To create a p-n junction, an area on the cap layer 640 isdiffused with a metal, for example, zinc to make the p-type region 680have, in one embodiment, a concentration in the range of 1×10¹⁶atoms/cm³ to 1×10¹⁹ atoms/cm³. In another embodiment, the concentrationof the p-type region is at least 1×10¹⁸ atoms/cm³. At the junction ofthe p-type region 680 and the n-type doped semiconductor layer 630, aP-N junction is formed.

In another embodiment p-type region 680 is created by diffusing zinc tothe InGaAs layer 630. In addition, the buffered layer 620 and cap layer640 may be InGaAs instead of InP as illustrated in FIG. 6.

Operationally, the diode 600 is reverse biased, causing a depletionregion to extend through the InGaAs intrinsic layer 630 to then-buffered layer 620. When light hits the position sensing detector 600,the light reaches the InGaAs absorption layer 630, where it is absorbedand charge carriers (holes and electrons) are generated by virtue of aphotovoltaic effect. As the p-type carriers drift towards the top InPlayer 640 and flow into the anode 650, the n-type carriers drift towardsthe InP substrate layer 610 and are collected by the cathodes 660 a, 660b, 660 c and 660 d (of which 660 c and 660 d are not shown in FIG. 6).The currents collected are measured and the position is determined inaccordance with methods well known in the art.

The manufacturing process of one embodiment of the position sensingdetector (PSD) of the present invention will now be described in greaterdetail. Persons of ordinary skill in the art should note that althoughone exemplary manufacturing process is described herein, variousmodifications may be made without departing from the scope and spirit ofthe invention.

Reference is now made to FIG. 6, which is a cross sectional view of oneembodiment of the PSD of the present invention, and FIGS. 7 a through 7q which are also cross-sectional views of the PSD of FIG. 6,illustrating exemplary manufacturing steps of the embodiment.Modifications or alterations to the manufacturing steps, theircorresponding details, and any order presented may be readily apparentto those of ordinary skill in the art. Thus, the present inventioncontemplates many possibilities for manufacturing the photodiode arrayof the present invention and is not limited to the examples providedherein.

Referring now to FIG. 7 a, in step 781, a device starting material 705is formed, and comprises semi-insulating substrate 710, which in oneembodiment, is InP. A buffered layer 720 is formed over thesemi-insulating substrate 710, which in one embodiment, is InP. Anabsorption layer 730, which in one embodiment is InGaAs, is then formedover the buffered layer 720. A cap layer 740 is then formed over theabsorption layer 730. The cap layer 740 is preferably n-type, and in oneembodiment, is formed from InP. While it is preferred that thesubstrate, buffered and cap layers 710, 720, 740 respectively are of InPwhile the absorption layer 730 be comprised of InGaAs, one of ordinaryskill in the art would appreciate that any suitable semiconductormaterial, which can be processed in accordance with the processing stepsof the present invention, may be used. In addition, device wafer layers710, 720, 730 and 740 are, in one embodiment, polished on both sides toallow for greater conformity to parameters, surface flatness, andspecification thickness. It should be understood by those of ordinaryskill in the art, however, that the above specifications are not bindingand that the type of material can easily be changed to suit the design,fabrication, and functional requirements of the present invention.

In step 782, as shown in FIG. 7 b, the device material 705 is subjectedto Plasma-Enhanced Chemical Vapor Deposition (PECVD) of silicon nitride745 at the front side. PECVD of silicon nitride is employed forpassivation to prevent oxidation of the exposed surface of the cap layer740.

In step 783, as shown in FIG. 7 c, a photoresist layer 746 is deposited,over the silicon nitride layer 745, to enable further etching of aspecific pattern on the surface of the device material 705. Generally,the photoresist layer 746 is a photosensitive polymeric material forphotolithography and photoengraving that can form a patterned coating ona surface. Thus, after selecting a suitable material and creating asuitable photoresist pattern, the thin photoresist layer 746 is appliedto the front side of device material 705. In one embodiment, thephotoresist layer is applied via a spin coating technique. Spin coatingis well known to those of ordinary skill in the art and will not bedescribed in detail herein.

Referring now to FIG. 7 d, at step 784, the device material 705 issubjected to p+ mask lithography on front side. P+ masking is employedto protect portions of device material 705. Generally, photographicmasks are high precision plates containing microscopic images ofpreferred pattern or electronic circuits. They are typically fabricatedfrom flat pieces of quartz or glass with a layer of chrome on one side.The mask geometry is etched in the chrome layer. In one embodiment, thep+ mask comprises a plurality of diffusion windows with appropriategeometrical and dimensional specifications. The photoresist coateddevice material 705 is aligned with the p+ mask. Light, such asultraviolet light, is projected through the mask, exposing thephotoresist layer 746 in the pattern of the p+ mask. The p+ mask allowsselective irradiation of the photoresist 746 on the device wafer 705.Photoresist area that are exposed to UV light will be removed whilethose that are shielded by the P+ mask are hardened by a subsequentpost-exposure development bake. Regions that are exposed to radiationare hardened while those that are reserved for diffusion remain shieldedby the p+ mask and easily removed. The exposed and remaining photoresistis then subjected to a suitable chemical or plasma etching process toreveal the pattern transfer from the mask to the photoresist layer 746.An etching process is then employed to remove the silicon nitride layer745 from the front side of the device material 705. In one embodiment,the pattern of the photoresist layer 746 and/or p+ mask defines regions747, on the front side, devoid of the nitride and photoresist layersdeposited in step 782 and ready for p+ diffusion.

Now referring to FIG. 7 e, in step 785, the front side of the device 705is subjected to p+ diffusion 747 followed by drive-in oxidation afterthe previous p+ masking and etching step 784. Generally, diffusionfacilitates propagation of a diffusing material through a host material.In step 785, an appropriate amount of dopant atoms, such as zinc, isdeposited onto the substrate device 705 and fills the gaps 747 left bythe removed photoresist and silicon nitride layers. Thereafter, in step786 of FIG. 7 f, exposed surfaces 742 are passivated with re-depositionof PECVD silicon nitride layer 745 followed by a coating of photoresistlayer 746.

As shown in FIG. 7 g, at step 787, the device material 705 is subjectedto mask lithography on front side for creating/opening anode windows750. In one embodiment, the anode-opening mask comprises a plurality ofdiffusion windows with appropriate geometrical and dimensionalspecifications. The photoresist coated device material 705 is alignedwith the anode-opening mask. A light, such as UV light, is projectedthrough the mask, exposing the photoresist layer 746 in the pattern ofthe anode-opening mask. The mask allows selective irradiation of thephotoresist 746 on the device wafer 705.

An etching process is then employed to remove the silicon nitride layer745 from the front side of the device material 705. In one embodiment,the pattern of the photoresist layer 746 and/or anode-opening maskdefines regions 750, on the front side, devoid of the nitride layer tobe used as anode windows 750. Thereafter, in step 788 of FIG. 7 h,exposed anode regions 750 and the entire front side are re-coated with anew layer 746 of photoresist.

At step 789, as shown in FIG. 7 i, anode metal lift-off mask lithographyis performed on the front side of the device wafer 705. As know topersons of ordinary skill in the art, the lift-off process allows forpatterning of the photoresist layer 746 before metallization or beforemetal is evaporated over the resist. Thus, the lift-off mask lithographyresults in patterned regions 751. This is followed by metallization 752on the front side of the device wafer 705 at step 790 of FIG. 7 j.

Referring now to FIG. 7 k, at step 791, the photoresist, of the earlierstep, is lifted off from the front side, exposing anti-reflective layer753, followed by a re-coat of the front side with a new layer ofphotoresist 746 at step 792 of FIG. 7 l.

As shown in FIG. 7 m, at step 793, the device wafer 705 is subjected tomask lithography on front side for creating/opening cathode windows 755.In one embodiment, the cathode-opening mask comprises a plurality ofdiffusion windows with appropriate geometrical and dimensionalspecifications. The photoresist coated device material 705 is alignedwith the anode-opening mask. A light, such as ultraviolet light, isprojected through the mask, exposing the photoresist layer 746 in thepattern of the cathode-opening mask. The mask allows selectiveirradiation of the photoresist 746 on the device wafer 705. Regions thatare exposed to radiation are hardened while those that are reserved fordiffusion remain shielded by the mask and easily removed. The exposedand remaining photoresist is then subjected to a suitable chemical orplasma etching process to reveal the pattern transfer from the mask tothe photoresist layer 746. An etching process is then employed to removethe silicon nitride layer 745 from the front side of the device material705. In one embodiment, the pattern of the photoresist layer 746 and/orcathode-opening mask defines regions 755, on the front side, devoid ofthe nitride layer to be used as cathode windows 755. Thereafter, in step794 of FIG. 7 n, exposed cathode regions 755 and the entire front sideare re-coated with a new layer 746 of photoresist.

At step 795, as shown in FIG. 7 o, cathode metal lift-off masklithography is performed on the front side of the device wafer 705. Asknow to persons of ordinary skill in the art, the lift-off processallows for patterning of the photoresist layer 746 before metallizationor metal is evaporated over the resist. Thus, the lift-off masklithography results in patterned regions 756. This is followed bycathode metallization 757 on the front side of the device wafer 705 atstep 796 of FIG. 7 p. Finally at step 797 of FIG. 7 q the photoresist,of the earlier step is lifted off from the front side of the devicewafer 705, exposing the anti-reflective layer 753.

FIG. 8 illustrates a plot of a plurality of test measurements ofposition sensing error obtained by using the PSD of the presentinvention. In one embodiment, test measurements are taken as a laserbeam is scanned across the active area of the photodiode. Both theactual mechanical position of the laser spot and the measured electricalposition are shown in FIG. 8. The position detection error representsthe deviation of electrical position from the mechanical position.

The average position detection error obtained from the test measurementdata represented in the plot 800 at a light wavelength of 1550 nm is0.118 mm in the horizontal plane (X-axis) and 0.116 in the verticalplane (Y axis).

Conventionally, tetra-lateral PSD have been manufactured using silicon.The use of InGaAs/InP in the present invention, however, enablesposition detection at eye safe wavelengths, namely wavelengths in theranging from 1.3-1.55 μm.

Since modifications can be made to the aforementioned constructionswithout departing from the scope of the invention, it is intended thatthe matter described be interpreted as illustrative rather thanrestrictive. For example, semiconductor materials other than InGaAs orInP including silicon and germanium may be used. Also, different dopingagents other than those mentioned above may be used while stillpreserving the diode structure and hence staying within the scope andintent of the present invention. The invention, therefore, should not berestricted, except to the following claims and their equivalents.

We claim:
 1. A position sensing detector comprising a photodiode havingan active area, said photodiode comprising a. A semi insulatingsubstrate layer; b. A buffered layer, wherein said buffered layer isformed directly atop the semi-insulating substrate layer; c. Anabsorption layer, wherein said absorption layer is formed directly atopthe buffered layer substrate layer; d. A cap layer, wherein said caplayer is formed directly atop the absorption layer; e. A plurality ofcathode electrodes electrically coupled to said buffered layer; and f.At least one anode electrode electrically coupled to a p-type region insaid cap layer, wherein said detector is capable of detecting eye-safewavelengths.
 2. The position sensing detector of claim 1 wherein saidbuffered layer comprises InP and wherein said eye-safe wavelengths arefrom 1.3-1.55 μm.
 3. The position sensing detector of claim 1 whereinsaid absorption layer comprises InGaAs.
 4. The position sensing detectorof claim 1 wherein said cap layer comprises InP.
 5. The position sensingdetector of claim 1 wherein the photo response non-uniformity is lessthan 1 micron across said active area.
 6. The position sensing detectorof claim 1 wherein the position detection error, in both the X and Ydirection is on the order of 100 μm across the active area.
 7. Theposition sensing detector of claim 1 wherein the photodiode comprisesfour cathode electrodes.
 8. The position sensing detector of claim 7wherein each individual cathode is positioned parallel to the othercathodes and in opposing corners of the photodiode.
 9. The positionsensing detector of claim 1 wherein the photodiode further comprises ananti-reflective layer positioned atop the cap layer.
 10. The positionsensing detector of claim 1 wherein said p-type region in said cap layeris formed by diffusing a region of said cap layer with a suitable dopantto create said p-type region.
 11. The position sensing detector of claim10 wherein said dopant is zinc.
 12. A position sensing detectorcomprising a photodiode having an active area, said photodiodecomprising a. A semi insulating substrate layer; b. A buffered layer,wherein said buffered layer is formed directly atop the semi-insulatingsubstrate layer; c. An absorption layer, wherein said absorption layeris formed directly atop the buffered layer substrate layer; d. A caplayer, wherein said cap layer is formed directly atop the absorptionlayer, wherein a p-n junction is formed between said cap layer and saidabsorption layer; e. A plurality of cathode electrodes electricallycoupled to said buffered layer; and f. At least one anode electrodeelectrically coupled to said cap layer.
 13. The position sensingdetector of claim 12 wherein said buffered layer comprises at least oneof InGaAs or InP.
 14. The position sensing detector of claim 12 whereinsaid absorption layer comprises at least one of InGaAs or InP.
 15. Theposition sensing detector of claim 12 wherein said cap layer comprisesat least one of InGaAs or InP.
 16. The position sensing detector ofclaim 12 wherein the position detection error, in both the X and Ydirection is on the order of 100 μm across the active area.
 17. Theposition sensing detector of claim 12 wherein the photodiode comprisesfour cathode electrodes.
 18. The position sensing detector of claim 12wherein said p-n junction is formed by diffusing a region of said caplayer with a suitable dopant.
 19. The position sensing detector of claim12 wherein said p-n junction is formed by diffusing a region of saidabsorption layer with a suitable dopant.
 20. A position sensing detectorcomprising a photodiode having an active area, said photodiodecomprising a. A semi insulating substrate layer; b. A buffered layer,wherein said buffered layer is formed directly atop the semi-insulatingsubstrate layer; c. An absorption layer, wherein said absorption layeris formed directly atop the buffered layer substrate layer; d. A caplayer, wherein said cap layer is formed directly atop the absorptionlayer, wherein a p-n junction is formed between said cap layer and saidabsorption layer; e. A plurality of cathode electrodes electricallycoupled to said cap layer; and f. At least one anode electrodeelectrically coupled to said cap layer.